1. Field of the Invention
The invention relates to a comprehensive two-dimensional gas chromatograph having a first separation column, a second separation column and a modulator controlled by a controller for sampling a chromatographic peak eluting from the first column into the second column.
The invention further relates to a modulator for such a chromatograph.
2. Description of the Related Art
A “Review of the Basic Concepts of Comprehensive Two-Dimensional Gas Chromatography” is provided by Ruby C. Y. Ong and Philip J. Marriott in Journal of Chromatographic Science, 40 (2002) 276-291.
Comprehensive two-dimensional gas chromatography (GC×GC) has to be distinguished from multidimensional or heart-cut gas chromatography (GC−GC), where two separation columns are connected by a valve that allows an introduction of a selected fraction (heart cut) of the eluate from the first column (first dimension) into the second column (second dimension) for further separation and detection. The selected fraction may be a chromatographic peak that emerges from the first column and contains analytes that have not yet been fully separated.
In comprehensive two-dimensional gas chromatography, the entire sample is ideally independently separated in the two separation columns that are connected sequentially with a modulator positioned between them. Typically, the first dimension is a conventional column and the second dimension is a short, fast-elution column. The modulator serves to focus the effluent from the first column in short plugs and to inject them in the second column for rapid separation. There are different types of modulators, some of them are based on temperature differences and others on valve operation.
In cryogenic modulators, for example, the analytes that appear at the end of the first column are collected and concentrated in a cooled region (trap) and released to the second column by heating this region. This allows the sample and transfer of complete chromatographic peaks eluting from the first column.
Valve-based modulators advantageously work without cooling and heating and can be used for fast peaks because they operate at a relatively high frequency. They are simpler, smaller, more rugged and less costly as thermal modulators. Valve-based modulators usually sample less than the complete chromatographic peak because they vent the effluent from the first column to a certain extent so that portions of the effluent will get lost. However, if the peak is short, it may be transferred as a whole by collecting the eluate coming from the first column into a sampling loop and then switching a valve and injecting the content of the sampling loop into the second column at a very high flow rate, a technique that is known for injecting a sample volume into a gas chromatograph. To this end, the modulator may include a six-port valve which, in a first position, connects the sampling loop in line with the first column and an exhaust and, in a second position, connects the sampling loop in line with a carrier gas source and the second column.
Following Gwen M. Gross et al. “High-Speed Gas Chromatography Using Synchronized Dual-Valve Injection”, Analytical Chemistry 76 (2004) 3517-3524 who disclose an injection system for high-speed gas chromatography, the above mentioned modulator can be modified by arranging a further six-port valve between the first valve and the second column. The second valve in its first position connects either the carrier gas source (first valve in first position) or the sampling loop (first valve in second position) to the second column. In its second position, the second valve connects the second column to the carrier gas source and the sampling loop to an exhaust (first valve in second position). Thus, it is possible to obtain a sharp, fast and accurate injection pulse that far exceeds the pulse width provided by a single valve. Because the high-speed valves actuate independently, the injection pulse width can be varied depending upon the separation requirements. However, high-speed six-port diaphragm valves are expensive, complex in design and may cause flow disturbances on switching.
From U.S. Pat. No. 6,447,581 B2 a gas flow switching device is known that allows valve-less injection of a sample into a separation column. The device comprises a planar component in which different gas passages are formed by micro-machining techniques in the broadest sense. A first gas passage is at one end connected to a carrier gas source via a capillary and a solenoid valve and at the other end connected to the separation column. A second gas passage receives at one end the sample via a capillary from a sampling loop and opens at the other end to an exhaust capillary. The first and second gas passages a connecting gas passage between the first and second gas passages communicate with one another via a connecting gas passage. When the solenoid valve is open, carrier gas flows through the first gas passage to the separation column, Here, a small percentage of the carrier gas flows into the connecting gas passage and keeps the sample flowing in the second gas passage away from the first gas passage and the column. When the solenoid valve is closed for a short time, the sample is diverted from the second gas passage into the connecting gas passage and reaches the separation column via the first gas passage.
As mentioned above, valve-based modulators and two-dimensional gas chromatographs vent the effluent from the first column to a certain extent so that portions of the effluent will get lost. In order to minimize the corrupting influence of the missing effluent, the peak from the first column is preferably sampled at several times into the second column so that several slices of each peak are delivered to the second column. This requires very fast switching and short delays, such as an injection pulse width of less than 3 ms and an injection cycle time of less than 9 ms.